The present invention relates to magnet wire insulation designed to withstand voltages present in inverter driven motors for a sustained period of time. More specifically, the present invention relates to magnet wire insulation that is intended to improve the life of motor windings when used in conjunction with an inverter drive, e.g., pulse-width modulated variable frequency drive.
Inverter drives and inverter driven motors have received increased attention because of continuing needs for greater energy efficiency. It has been estimated that three-phase induction motors consume 60-70% of the electrical energy used in the United States. These motors obviously waste substantial energy when run at full speed when conditions do not require it.
An adjustable speed drive (ASD) allows a motor to operate at variable speed by providing variable frequency to the motor. Electronic ASDs convert the incoming line voltage at 60 Hz to direct current (DC). The inverter then generates variable frequencies as input to the motor. These variable frequencies, however, can exhibit steep wave shapes that have been linked to premature motor winding failures in 440+voltage motors. The mode of failure in these motor windings has been linked to the degradation of the wire insulation caused by the high voltage and higher frequency wave shapes.
Various attempts have been made to reduce premature failures as a result of degradation of the wire insulation. These attempts have included minimizing damage to the wire and insulation during handling and manufacture of the motors, and using shorter lead lengths from the inverter to the motor where appropriate. Further, a reactor coil or a filter between the inverter drive and the motor can extend the life of the windings by reducing the voltage spikes and high frequencies generated by the inverter drive/motor combination. However, such coils are expensive and add to the overall cost of the system. Increasing the amount of insulation from standard heavy build magnet wire can improve the life of the windings in the motor, but this option is both expensive and decreases the amount of space for the copper in the motor, thereby producing a less efficient motor. Another option includes increasing the amount of varnish in the motor windings, however, this strategy is ineffective if the windings are not completely covered.
Therefore, there is a need for a magnet wire insulation that is designed to withstand voltages which are present in inverter driven motors for longer periods of time as compared to the present constructions.
The disclosed invention improves the resistance to voltages present in the windings of inverter driven motors without many of the drawbacks associated with prior strategies for reducing premature failures. The invention includes adding high surface area silica or a mixture of silica and chromium oxide to the magnet wire insulation. Thus, the disclosed insulation comprises one or more layers of a cured, wire enamel type polymer with high surface area silica or a mixture of silica and chromium oxide dispersed in one or more of the polymeric layers.
The invention extends the life of the windings in a motor used in an inverter drive application. In practicing the invention, a copper conductor is overcoated with a polyester base coat and a polyamide-imidc topcoat. High surface area silica or a blend of silica and chromium oxide is added to. the polyamide-imide topcoat. The invention, however, is not limited to two-layer enamels used in magnet wires.
While the prior art shows the addition of inorganic oxides or organo-metallic compounds to magnet wire enamels, the life of motor windings used in inverter drives and inverter drive motors is improved when high surface area, i.e., fumed silica, is dispersed either alone or with chromium oxide in the insulation. The present invention provides an inorganic oxide, namely fumed silica, which has a large surface area for permitting more energy dissipation in the insulation. This improves the life of motor windings that are subjected to the high voltages present in inverter driven motors. Moreover, a mixture of fumed silica and a low resistivity oxide, namely chromium oxide, provides additional improvement to the life of motor windings. Though not bound by any particularly theory, it is believed that the larger surface area provided by fumed silica will permit more energy dissipation in the insulation and the low resistivity oxide will spread the electrical charge over the surface of the insulation. It has been discovered that the mixture of fumed silica and chromium oxide provides a better result than using either the fumed silica or chromium oxide alone. The addition of a third, unfilled topcoat does not significantly affect the life of motor windings, but helps decrease tooling wear associated with the abrasive inorganic oxide particles.
It was intended that one of the inorganic oxide additives have a large surface area for permitting more energy to dissipate in the insulation. Silica is believed to be the only inorganic oxide commercially available in grades having different particulate surface areas. Available specific surface areas for silica range from approximately 90 to 550 m2/g. Since it was found that resistance to insulation failure in an inverter drive motor improves with increasing silica surface area, the preferred silica grade for the present invention has a specific surface area between about 380 and 550 m2/g. The preferred range of silica in the insulation is between about 10 and 50% based on weight. Substantial improvement is not observed at silica levels below about 10%, and insulation flexibility is lost at silica levels greater than about 50%. Further, to ensure a smooth, continuous surface, the silica is milled to break up any agglomerates. The silica may be milled directly into the wire enamel in the presence of solvent, or the silica can be milled in solvent and then added to the enamel. In either case, milling breaks up the agglomerates and the solvent keeps the particles from re-agglomerating. Once the silica has been dispersed in the polymer, the polymer is applied to a conductor in a conventional fashion. For magnet wire, the uncured insulation is applied by using multi-pass coating and wiping dies followed by curing at elevated temperature.
As set forth previously, even better results in extending the life of the windings in a motor is achieved by adding a mixture of inorganic oxides, namely silica and chromium oxide, into the magnet wire insulation. A dispersion of silica and chromium oxide is added to a polyamide-imide wire enamel whereby the concentration of the total oxide is in the range of about 5-50% based on the total polymer weight. The resulting magnet wire enamels may then be coated on a wire using conventional techniques.